SCSI Technology Primer

SCSI-3

If SCSI-2 seemed like a variation around two themes, then SCSI three is going to feel a lot like Taco Bell to you. That is, SCSI three is essentially a handful of ingredients combined in a variety of different ways. Unlike Taco Bell, I would suggest SCSI-3 actually produces new and interesting things. Rather than cover every possible permutation of SCSI-3 in narrative, let's hit the high-points and then take a look at a handy table. SCSI-3 is all about bus speed. Whereas we hit a 20MHz barrier with SCSI-2, SCSI-3 is reaching 80 MHz in practice, and 160 MHz in theory. SCSI-3 is indeed a very broad category when one looks at products: bus speeds range from 20-160 MHz, and throughput ranges from 40-320 MB/s!

The following table lists each type of SCSI technology and its specs. This table is derived from a similar table at the SCSI Trade Association website. The numbers in parentheses denote a comment below the table.

STA Terms

Clock Speed (MHz)

Bus Speed, MBytes/Sec. Max

Bus Width (in bits)

Max. Bus Lengths, meters (1)

Max. Device Support

Single-ended

LVD

HVD

SCSI-1 (2)

5

5

8

6

(3)

25

8

Wide SCSI (2)

5

10

16

3

(3)

25

8

Fast SCSI (2)

10

10

8

3

(3)

25

8

Fast Wide SCSI

10

20

16

3

(3)

25

16

Ultra SCSI (2)

20

20

8

1.5

(3)

25

8

Ultra SCSI (2)

20

20

8

3

-

-

4

Wide Ultra SCSI

20

40

16

-

(3)

25

16

Wide Ultra SCSI

20

40

16

1.5

-

-

8

Wide Ultra SCSI

20

40

16

3

-

-

4

Ultra2 SCSI (2,4)

40

40

8

(4)

12

25

8

Wide Ultra2 SCSI (4)

40

80

16

(4)

12

25

16

Ultra3 SCSI (6) (AKA ULTRA 160)

80

160

16

(4)

12

(5)

16

Ultra320 SCSI

160

320

16

(4)

12

(5)

16

(1) The listed maximum bus lengths may be exceeded in Point-to-Point and engineered applications.
(2) Use of the word "Narrow", preceding SCSI, Ultra SCSI or Ultra2 SCSI is optional.
(3) LVD (Low Voltage Differential) was not defined in the original SCSI standards for this speed. If all devices on the bus support LVD, then 12-meters operation is possible at this speed. However, if any device on the bus is single-ended only, then the entire bus switches to single-ended mode and the distances in the single ended column apply.
(4) Single-ended is not defined for speeds beyond Ultra.
(5) HVD (High Voltage Differential, or Differential) is not defined for speeds beyond Ultra2.
(6) After Ultra2, all new speeds are wide only.

So, what exactly do all of the numbers mean? Unless you know what form of SCSI you're using, they won't mean anything. For new users, the most important thing to find out is what kind of SCSI Controller (also called a host adapter) is in your box. Depending upon what form of SCSI that card supports and how much cabling you have, you can have anywhere from 4 to 15 devices hooked up to it (Note: 15 is correct as your host adapter also counts as a device on the SCSI bus). And, as the table demonstrates, there are instances where even if you have a certain type of SCSI controller, the most easily over-looked circumstances (such as cable length!) can greatly reduce the number of devices you can have attached to the SCSI bus. To understand why this happens, you need to know what the different types of SCSI
are and what differentiates them from their brethren.

For example: on the table above there are only two notable differences between SCSI-1 and Fast SCSI. These are the Bus Speed in MB/sec and the cable length for single-ended devices. What changed? SCSI-1 allowed for 5MHz timing on the SCSI bus while Fast SCSI allows for 10MHz timing on the SCSI bus and therefore the theoretical maximum is doubled. Fast Wide SCSI increases the bus width to 16 bits from 8 bits, gaining another doubling in the maximum bus throughput. This same maxim has held true as the newer forms of SCSI came about ... change the timing on the SCSI bus, make sure the cables meet the requirements for those changes and increase the bus width to 8, 16 or 32 bits to gain more performance. These are not the only changes that have been made between each of the various generations of
SCSI, but they are some of the most significant in terms of performance increases. Other changes have included additions to the SCSI command set to allow for things like tape drives, CD-ROM devices, scanners and various other peripherals. The following is a list of some of the more common forms of SCSI technology and a definition of each:

While these are not all of the different variations of SCSI, they are some of the most frequently encountered forms of it. The same sorts of changes (bus width, cable type, bus timing) are all present in the newer forms of SCSI as well and the table above gives a pretty good indication of what has been changed in order to gain the speeds that are listed for each form. Along with the various speed improvements, different command set additions have been made over time as well. For a complete listing, the ANSI committee responsible for SCSI, T10, has a comprehensive website.

Tidbits from the future: SAS and iSCSI

Imagine being able to use the cutting-edge of SCSI technology through your good old, trusty, and useless serial port. In short, that's what Serial-attached SCSI, aka SAS, is all about. SAS is aimed at the middle-ground between high-end Fibre Channel solutions and "lower end" IDE/ATA based solutions. SAS promises to be able to utilize ATA and SCSI devices in an overarching SCSI configuration that takes advantage of the ubiquitous serial connection for increased chain length and signal strength. Of course, there's already an IDE/Serial solution akin to this known as SerialATA. But as proponents of SAS will point out, many IT shops would prefer to be able to leverage their existing SCSI investments rather than move to the IDE/ATA platform. How true this is won't be known until SAS hits the scenes, which is probably still at
least a year away. For more information on SAS, check out this recently released (5/27/2002) T10 whitepaper (PDF). For what it's worth, some motherboards will be shipping with SerialATA support by the end of this summer (e.g., Soyo).

iSCSI is SCSI over IP Networks/Ethernet. The SCSI commands and data get encapsulated in Ethernet or TCP/IP packets, which obviously allows for greater distances between devices, as well as potentially making the storage devices accessible from a number of clients simultaneously. The problem is that to really make it practical, you need a great deal of bandwidth--1Gbps Fiber is only capable of handling around 80-100MB/sec. Industry consensus is that iSCSI's future hinges on the advent of 10Gbps fiber. Nevertheless, Adaptec already sells NICs that do iSCSI, and they have a whitepaper about it on their site. iSCSI may blossom before 10Gbps Fibre is available on account of the
increasing popularity of Storage Area Networks (SANs).